how to adjust the network analysizer sweep power?

Hi Guys
I need to test the impedance of a system using network analysizer. However, the input power for the system can not be exceed -30 dBm. I believe when you measure the S parameter, the network analysizer sends a signal at different frequency with some amount of power and measured the reflected power to calculate the impedance. Can anyone tell me how to adjust the amount of power level that network analysizer send?
I am using network analysizer from agilent, the model is N5230A.
Thanks

There is a button for power setting and type in -30dBm power.

Don't forget..!
If you calibrate with high power and then if you decrease the power, calibration will not be valid anymore.
In the other hand, calibartion with this so low power will be more inconsistent or uncertain...
So, be careful about constraints..